One of the objectives in the separation of multicomponent feeds in a distillation column utilizing bubble trays is to effect uniform and intimate vapor contact with the liquid on the tray. Bubble-cap type trays are preferred where low vapor rates must be accommodated since they are not subject to weeping as are sieve trays or valve trays, which substantially reduces mass transfer efficiency. In order to achieve uniform and intimate vapor contact it is known that vapor flow through the bubble caps should be substantially uniform in order to achieve the maximum mass transfer efficiency.
One of the problems associated with bubble-cap trays in distillation columns is instability at low vapor rates. In some cases where the vapor load to the tray is extremely low there may be instances where there is vapor flow through a small portion of the caps and little to no vapor flow through the other caps on the tray. When this condition occurs, liquid bypasses the caps which have vapor flow and avoids vapor contact, resulting in very loss mass transfer efficiency. Instability often is caused by varying pressure seals above the caps on a tray due to the hydraulic gradient inherently caused by the flowing liquid, or due to unlevelness of the tray.
Another problem in the operation of distillation columns has been associated with the distillation of multicomponent feeds where the vapor and liquid loads vary over a wide range. Vapor loads to the trays in the column can vary because of changing concentrations of more volatile component in the feed or alternatively because of variations in the feed flow rate to the column. Liquid loads to the tray will also vary because of changing feed composition or feed rates. Bubble-cap trays normally are limited to design turndown ratios of about 4-8:1 (the ratio of the volume of vapor passing through the cap at design velocity compared to the volume of vapor passing through the cap at minimum acceptable velocity and the ratio of the volume of liquid at design rate to the volume at minimum rate). The following are representative turndown ratios of other distillation devices: packed columns 3:1, sieve trays 2-4:1, and valve trays, 4:1. These columns normally have more limited turndown ratios than bubble-cap trays. Generally, the turndown ratio implies equal turndown for both liquid and vapor on a tray. Such a result is not normally the case, however, with changing feed compositions.
Several techniques have been developed as shown by the prior art to correct instability of trays, e.g. the instability being caused by the hydraulic gradient across the bubble tray or unlevelness. In an article "Optimum Bubble Cap Tray Design" by W. L. Bolles, Petroleum Processing (March, 1956), page 89, (see also U.S. Pat. No. 2,699,929 and U.S. Pat. No. 2,692,128), the author suggests the use of stepped-level bubble caps to correct tray instability. In the Bolles design, the bubble caps are positioned at various levels above the tray so that the bubble cap elevation below the surface of the liquid component is substantially the same across the tray. By matching the height of the bubble cap to the liquid head, the pressure seal over the bubble caps is substantially equal, if vapor and liquid rates are maintained at essentially the same ratio.
In the continuation of the Bolles article in Petroleum Processing (April, 1956), page 75, the author suggests that tray instability also can be corrected at low vapor loads by blanking off some rows of caps on the bubble tray where such low vapor loads are anticipated.
In a text entitled Unit Operations of Chemical Engineering by McCabe and Smith (1956), page 734-735, the authors suggest a procedure similar to Bolles to correct plate instability in large columns. This is accomplished by causing the liquid to flow only across a portion of the plate.
U.S. Pat. No. 2,871,003 discloses a valve-type bubble cap having a built in vapor flow adjuster. The cap utilizes a diaphragm to govern the amount of vapor leaving the cap. Ports of variable size are incorporated into the cap and the diaphragm opens or closes the ports depending upon the vapor pressure. Sticking or freezing of the diaphragm would result in weeping and reduced efficiency since the caps have no risers.
It is also known that trays incorporating bubble caps have been designed to handle a wide range of vapor loads, e.g. turndown ratios of 10-60:1 due to changes in feed composition or feed rates to the column. One of the known techniques is to establish a plurality of separation zones on a tray with each separation zone having a distinct liquid head, i.e. a distinct and substantially equal pressure seal above the bubble caps in that separation zone. Each separation zone has a pressure seal or head different from the pressure seal or head in another zone. The magnitude of the pressure seal in each separation zone is preselected, usually greater than the hydraulic gradient or unlevelness, so that at low vapor velocities only one or perhaps two separation zones on the tray are operable. As the vapor load to the tray is increased, other separation zones become operable. Two techniques for zone formation as described are suggested. One technique is to section the tray by placing weirs of different height on the tray and perpendicular to liquid flow. Since the height of each weir is different, the liquid level and pressure seal in each zone is different from another separation zone. The creation of separation zones of different liquid level creates zones of different pressure seals. At the lowest vapor load, then, only that zone with the smallest pressure seal is operable. Since the zone with the smallest pressure seal must necessarily be at the liquid outlet, tray leakage will result in bypassing of the active zones.
Another technique for zone formation involves setting the liquid level on the tray at a predetermined level and then adjusting the height of the bubble caps above the floor. However, instead of matching the height of the liquid gradient as in Bolles, the height of the bubble caps below the liquid level is varied much more dramatically so that the pressure seals are substantially different. One difficulty with the stepped level arrangement of bubble caps along the tray is that the resistance to liquid flow across the tray is increased and the increased resistance exacerbates the liquid gradient. Greater power may be required because of the increased resistance imparted to liquid flow. A second disadvantage of both techniques is that the substantial difference in pressure seals results in significant imbalance in vapor flow over the entire operating range of the tray, with a corresponding reduction in mass transfer efficiency in the higher range of operating rates.
Another technique which has been used to enhance tray stability incorporates a perforated inlet weir to initiate frothing. However, at low vapor rates such weirs exhibit substantial weeping through the perforations resulting in substantial loss of mass transfer efficiency.
The use of bubble caps having an elevated frontal skirt portion has been suggested but these too are affected by tray levelness and result in reduced mass transfer efficiencies at higher operating rates.